Capacitors and supercapacitors explained

Scientists have been struggling to develop energy storage solutions such as batteries and capacitors that can keep up with the current rate of electronic component evolution for a number of years. Unfortunately, the situation we are in now is that while we are able to store a large amount of energy in certain types of batteries, those batteries are very large, very heavy, and charge and release their energy relatively slowly. Capacitors, on the other hand, are able to be charged and release energy very quickly, but can hold much less energy than a battery. Graphene application developments though have lead to new possibilities for energy storage, with high charge and discharge rates, which can be made very cheaply. But before we go into specific details, it would be sensible to first outline the basics of energy storage and the potential goals of developing graphene as a supercapacitor.

Capacitors and supercapacitors explained

A capacitor is an energy storage medium similar to an electrochemical battery. Most batteries, while able to store a large amount of energy are relatively inefficient in comparison to other energy solutions such as fossil fuels. It is often said that a 1kg electrochemical battery is able to produce much less energy than 1 litre of gasoline; but this kind of comparison is extremely vague, mathematically illogical, and should be ignored. In fact, some electrochemical batteries can be relatively efficient, but that doesn’t get around the primary limiting factor in batteries replacing fossil fuels in commercial and industrial applications (for example, transportation); charge time.

High capacity batteries take a long time to charge. This is why electrically powered vehicles have not taken-off as well as we expected twenty or thirty years ago. While you are now able to travel 250 miles or more on one single charge in a car such as the Tesla Model S, it could take you over 43 hours to charge the vehicle using a standard 120v wall socket in order to drive back home. This is not acceptable for many car users. Capacitors, on the other hand, are able to be charged at a much higher rate, but store (as already mentioned) somewhat less energy.

Supercapacitors, also known as ultracapacitors, are able to hold hundreds of times the amount of electrical charge as standard capacitors, and are therefore suitable as a replacement for electrochemical batteries in many industrial and commercial applications. Supercapacitors also work in very low temperatures; a situation that can prevent many types of electrochemical batteries from working. For these reasons, supercapacitors are already being used in emergency radios and flashlights, where energy can be produced kinetically (by winding a handle, for example) and then stored in a supercapacitor for the device to use.

A conventional capacitor is made up of two layers of conductive materials (eventually becoming positively and negatively charged) separated by an insulator. What dictates the amount of charge a capacitor can hold is the surface area of the conductors, the distance between the two conductors and also the dielectric constant of the insulator. Supercapacitors are slightly different in the fact that they do not contain a solid insulator.

Instead the two conductive plates in a cell are coated with a porous material, most commonly activated carbon, and the cells are immersed in an electrolyte solution. The porous material ideally will have an extremely high surface area (1 gram of activated carbon can have an estimated surface area equal to that of a tennis court), and because the capacitance of a supercapacitor is dictated by the distance between the two layers and the surface area of the porous material, very high levels of charge can be achieved.

While supercapacitors are able to store much more energy than standard capacitors, they are limited in their ability to withstand high voltage. Electrolytic capacitors are able to run at hundreds of volts, but supercapacitors are generally limited to around 5 volts. However, it is possible to engineer a chain of supercapacitors to run at high voltages as long as the series is properly designed and controlled.

Graphene-based supercapacitors

Supercapacitors, unfortunately, are currently very expensive to produce, and at present the scalability of supercapacitors in industry is limiting the application options as energy efficiency is offset against cost efficiency. This is the reason why a paper by researchers at the UCLA has been so highly referred to within scientific circles and publications as they were able to produce supercapacitors made out of graphene by using a simple DVD LightScribe writer on a home PC. This idea of creating graphene monolayers by using thermo lithography is not necessarily a new one, as scientists from the US were able to produce graphene nanowires by using thermochemical nanolithography back in 2010; however, this new method avoids the use of an atomic force microscope in favour of a commercially available laser device that is already prevalent in many homes around the world.

Why are scientists looking at using graphene instead of the currently more popular activated carbon? Well, graphene is essentially a form of carbon, and while activated carbon has an extremely high relative surface area, graphene has substantially more. As we have already highlighted, one of the limitations to the capacitance of ultracapacitors is the surface area of the conductors. If one conductive material in a supercapacitor has a higher relative surface area than another, it will be better at storing electrostatic charge. Also, being a material made up of one single atomic layer, it is lighter. Another interesting point is that as graphene is essentially just graphite, which is a form of carbon, it is ecologically friendly, unlike most other forms of energy storage.

The efficiency of the supercapacitor is the important factor to bear in mind. In the past, scientists have been able to create supercapacitors that are able to store 150 Farads per gram, but some have suggested that the theoretical upper limit for graphene-based supercapacitors is 550 F/g. This is particularly impressive when compared against current technology: a commercially available capacitor able to store 1 Farad of electrostatic energy at 100 volts would be about 220mm high and weigh about 2kgs, though current supercapacitor technology is about the same, in terms of dimensions relative to energy storage values, as a graphene-based supercapacitor would be.

The future for graphene-based supercapacitors

Due to the lightweight dimensions of graphene based supercapacitors and the minimal cost of production coupled withgraphene’s elastic properties and inherit mechanical strength, we will almost certainly see technology within the next five to ten years incorporating these supercapacitors. Also, with increased development in terms of energy storage limits for supercapacitors in general, graphene-based or hybrid supercapacitors will eventually be utilized in a number of different applications.

Vehicles that utilize supercapacitors are already prevalent in our society. One Chinese company is currently manufacturing buses that incorporate supercapacitor energy recovery systems, such as those used on Formula 1 cars, to store energy when braking and then converting that energy to power the vehicle until the next stop. Additionally, we will at some point in the next few years begin to see mobile telephones and other mobile electronic devices being powered by supercapacitors as not only can they be charged at a much higher rate than current lithium-ion batteries, but they also have the potential to last for a vastly greater length of time.

Other current and potential uses for supercapacitors are as power backup supplies for industry or even our own homes. Businesses can invest in power backup solutions that are able to store high levels of energy at high voltages, effectively offering full power available to them, to reduce the risk of having to limit production due to inadequate amounts of power. Alternatively, if you have a fuel cell vehicle that is able to store a large amount of electrical energy, then why not use it to help power your home in the event of a power outage?

We can expect that this scenario of using advanced energy storage and recovery solutions will become much more widely used in the coming years as the efficiency and energy density of supercapacitors increases, and the manufacturing costs decrease. While graphene-based supercapacitors are currently a viable solution in the future, technology needs to be developed to make this into a reality. But rest assured, many companies around the world are already trialling products using this technology and creating new ways to help subsidise the use of fossil-fuels and toxic chemicals in our ever-demanding strive for energy.